In Elorus you cannot add multiple branches per contact. You can only designate a shipping address different from the billing address (headquarters).
However, there is an alternative way to create client branches. You can create separate contacts with the same billing address but different delivery addresses. In order to distinguish them you can include something specific in their Name and Surname fields (such as Branch No1 or London Branch).
When creating a new document, you can type "London branch" in the client field and the system will retrieve its information.
Also, if you do not want to show this information on your documents (the branch's name, that is) you can hide it:
Per document, by clicking the Content option and deselecting the "Client's personal information" box.
For all your documents, through your templates settings. Edit you template and deselect the "Client's personal information" box inside the "Content" tab..
By creating separate contacts, you won't be able to have a single contact statement to keep track of balances. In case this is important to you, you can always type the delivery address in the "Public notes" when adding a new document.